I don't have one in my arsenal or have one for sale, but I have used them before.
They work just like the larger RCBS powder measure except instead of having an screw adjustable powder chamber, you interchange rotors with fixed volume chambers to set up for the load you're wanting to throw.

They are a nice little system if you have one or two pet loads that you happen to find the correct rotor for.
Otherwise, if you want a Little Dandy plus all, or even several, of the rotors it'll get pretty spendy.
Consider that each rotor is just over $10 and there are 28 in the full set you're possibly looking at up to $300 to get the full set...plus around $30 for the measure itself. These are new prices...you might find used stuff cheaper.

It's about the same as buying a set of Lee dippers...you've got a set of fixed volume chambers to work with and need to correlate your loads around those choices.

For less money you can get a regular RCBS, or any other brand, adjustable measure with the small chamber (or a combo unit with both large and small chambers that will also measure rifle loads).